Voice communications over the public switched telephone network (PSTN) have traditionally been limited in bandwidth to the frequency range of 300-3400 kHz. New networks for voice communications, such as cellular telephony and voice over IP (VoIP), may not have the same bandwidth limits, and it may be desirable to transmit and receive voice communications that include a wideband frequency range over such networks. For example, it may be desirable to support an audio frequency range that extends down to 50 Hz and/or up to 7 or 8 kHz. It may also be desirable to support other applications, such as high-quality audio or audio/video conferencing, that may have audio speech content in ranges outside the traditional PSTN limits.
Extension of the range supported by a speech coder into higher frequencies may improve intelligibility. For example, the information that differentiates fricatives such as ‘s’ and ‘f’ is largely in the high frequencies. Highband extension may also improve other qualities of speech, such as presence. For example, even a voiced vowel may have spectral energy far above the PSTN limit.
In conducting research on wideband speech signals, the inventors have occasionally observed pulses of high energy, or “bursts”, in the upper part of the spectrum. These highband bursts typically last only a few milliseconds (typically 2 milliseconds), with a maximum length of about 3 milliseconds, may span up to several kilohertz (kHz) in frequency, and occur apparently randomly during different types of speech sounds, both voiced and unvoiced. For some speakers, a highband burst may occur in every sentence, while for other speakers such bursts may not occur at all. While these events do not generally occur frequently, they do seem to be ubiquitous, as the inventors have found examples of them in wideband speech samples from several different databases and from several other sources.
Highband bursts have a wide frequency range but typically only occur in the higher band of the spectrum, such as the region from 3.5 to 7 kHz, and not in the lower band. For example, FIG. 1 shows a spectrogram of the word ‘can’. In this wideband speech signal, a highband burst may be seen at 0.1 seconds extending across a wide frequency region around 6 kHz (in this figure, darker regions indicate higher intensity). It is possible that at least some highband bursts are generated by an interaction between the speaker's mouth and the microphone and/or are due to clicks emitted by the speaker's mouth during speech.